(1) Field of the Invention
This invention relates to a hydrophilic copolymer, a process for producing the copolymer, a photosensitive composition comprising the copolymer, and a photosensitive rubber plate made from the copolymer. More particularly, it relates to a photosensitive composition having an improved processability, a photosensitive rubber plate capable of being washed with water at a high rate at the step of development, a hydrophilic copolymer used for the preparation of the photosensitive composition and the photosensitive rubber plate, and a process for producing the hydrophilic copolymer.
(2) Description of the Related Art
A flexographic printing plate is made by a process wherein an original film is closely adhered on a photosensitive surface of a photosensitive rubber or resin plate, the film-adhered photosensitive surface is irradiated with an actinic radiation so that part of the photosensitive surface is exposed to the actinic radiation, and the unexposed area is removed by washing to form a relief. It is generally required that a photosensitive material used for the preparation of the photosensitive rubber plate for a flexographic printing plate can be washed at a high rate so that the relief is easily formed.
As a typical example of the photosensitive material, there can be mentioned a photosensitive elastomer composition comprising an elastomer such as a styrene-butadiene block copolymer or a styrene-isoprene block copolymer and a carboxyl group-containing hydrophilic copolymer as the major ingredients, and a photopolymerizable ethylenically unsaturated monomer. This photosensitive elastomer composition has a problem such that the resulting photosensitive rubber plate cannot be washed at a high rate.
To cope with the above-mentioned problem, i.e., provide a photosensitive elastomer composition capable of being washed at a high rate, it has been proposed in WO 94/-23342 to incorporate in a photosensitive elastomer composition a phosphate ester group-containing copolymer as a hydrophilic copolymer comprising phosphate ester-containing monoethylenically unsaturated monomer units as the indispensable ingredient. This photosensitive elastomer composition has good and balanced transparency and strength, as well as enhanced washability at a high rate.